Plant and insect fragments
In many cases particles are impossible to identify since they show no recognizable features. When their EDS spectra show predominantly carbon and oxygen it is reasonable to suspect their origin being from organic material. Particles of this kind are common in settled dust and in ambient air samples. In some cases unique structures are discernible which makes it possible to recognize them. Examples are the particles depicted below. Figures 1-4 show hair-like structures called trichomes that are located on leaves, stems or other parts of the plant body. Trichomes are single- or multi-celled outgrowths from the epidermis of the plant body. Some plant species appear to be devoid of trichomes, while others have leaves or stems densely covered with hairs. When the plant die and wither, these trichomes may detach and become airborne. The Cornus leaf hairs (Fig 1) showed a high content of calcium carbonate and traces of magnesium as well, as illustrated in Fig 1B. Sometimes fragments of dead insects can be found in indoor dust samples. Their chitinous exoskeleton may have hair or different kind of outgrowths that are recognizable in the microscope (Fig 9 – 15). In the SEM, which provides high magnification, high resolution and three-dimensional perspective, the plant and insect fragments often show eye-catching structures. Symmetry is very common in nature. Also on micro-scale, looking at parts or fragments of organisms, there seems to be a balanced distribution of duplicate parts. Examples are shown in Fig 5 and 9.
Insect excreta
Particles like those imaged in Fig 16 and 19 have previously been interpreted as spores of actinomycetes. They are however found to be secretory granules produced by some insects (leafhoppers) and are given the scientific term brochosomes. These granules have the function to make the insect water repellent. The particles may easily become airborne. Most often they are found in clusters, as in Fig 16. Fig 17 shows a cluster of brochosomes on an aphid and on a leafhopper in Fig 18 and 19. You can find a lot of information on brochosomes on the website of Roman Rakitov at Illinois Natural History Survey